Search

The Es1 sandstone is a fruitful hydrocarbon zone of Eocene Shahejie Formation. Therefore, the present study comprises the subsurface facies analysis established on the description of seven wells. Methods include core observation, thin section petrography, scanning electron microscope (SEM), cathodoluminescence (CL), and X-ray diffraction to evaluate the microfacies, lithofacies, and rock characteristics. Two microfacies of lithofacies have been established as sandstone microfacies and mudstone microfacies on the basis of grain size. Among them, sandstone is further divided into four sub-microfacies on the basis of microscopic studies, rock fragments and mineral composition, which are arkose sandstone, lithic arkose, feldspathic litharenite, and litharenite. These microfacies consisting quartz, feldspar as abundant detrital grains followed by rock fragments and micas, whereas calcite, ferro-calcite, quartz overgrowth, and clay are the main cement. The Es1 sandstone consists of good primary porosity and permeability as well as good secondary porosity caused by fracturing and dissolution of unstable minerals that enhance the reservoir quality. Whereas cementation and compaction reduce the reservoir characteristics. Studied formation divided into five lithofacies including conglomerate sandstone lithofacies, medium to coarse-grain sandstone lithofacies, fine sandstone lithofacies, siltstone lithofacies, and mudstone lithofacies. Core observation, sedimentary structures, facies sequences of the well log, and sedimentary composition reveal that Es1 deposited into the lacustrine delta fan depositional environment. Rock grain size indicates that Es1 contains mixtures of grains including pebbles, sand-size as well as minor silt and clay size. Grain size varies from conglomerate to clay size. The Es1 sandstone was deposited in braided river (conglomerates sandstone), channel bar, lacustrine, fluvial channel (massive bedding, cross-bedding, and graded bedding sandstone), deltaic, and flood-plain (siltstone and mudstone) sedimentary environment. It is concluded that higher transport energies in channels deposited coarse-grain sediments in shallowest part of the lake setting. However, the energy decreases towards greater depth which deposited mudstone in central part of the lake. [ABSTRACT FROM AUTHOR]

This paper discusses the effect of the source rock–reservoir assemblage and the assemblage–induced variability in hydrocarbon expulsion and the subsequent molecular composition as well as molecular thermal maturity within a hybrid shale system. The characterization work was conducted on lithofacies and microlamina scales using core samples from the Chang 7 3 sub-member of the Triassic Yanchang Formation in the Ordos Basin, China. Samples were collected from a narrow interval with a depth range of less than 15 m, and the main characterization work was performed by Rock-Eval pyrolysis and GC–MS analysis. The results show that chemical fractionation of preferential expulsion and migration of the saturated fraction exists in the source rock–reservoir assemblages at both the lithofacies and lamina scales. However, the molecular composition behaves differently at the lithofacies and lamina scale's source rock–reservoir assemblages, in which ∑C 21 − /∑C 22+ is higher in lamina scale reservoir but lower in lithofacies scale reservoir. It is assumed that the low-molecular weight n -alkanes also follow molecular fractionation. The lithofacies reservoir has a lower ∑C 21 − /∑C 22+ because of the strong storage capacity of the laminated micro-reservoir within shale, which prevents the newly generated lighter oil from being charged into the lithofacies reservoir. The variation trends of thermal maturity indices Ts/hopane, the relative pregnane content, and TA(I)/TA(I + II) ratios, which have the same chemical basis with ∑C 21 − /∑C 22+ , carry the same maturity signature as ∑C 21 − /∑C 22+. The above profile of the molecular composition and molecular-derived thermal maturity parameters indicate that within the short interval of a shale system where no differences in thermal maturity are expected, chain scission reactions and their derived thermal maturity indicators are very sensitive to source rock and reservoir. In addition, within a shale system, oil is more easily to expel out from the organic-rich lithofacies that are interbedded with organic-lean lithofacies. Oil expulsion may promote both chain cracking of oil and subsequently kerogen decomposition. This may provide geological evidence to explain why the frequent-stacking assemblage of source rock and reservoir lithofacies in a hybrid shale system is an ideal target for shale oil exploration. [ABSTRACT FROM AUTHOR]

Facies identification is a fundamental issue on evaluation and prediction for siliciclastic sediments. The purpose of this paper is to identify sedimentary-diagenetic facies and investigate the influence of sedimentary-diagenetic facies on reservoir quality in coarse-grained deposits from the Eocene Es4 interval, Dongying Depression, Bohai Bay Basin. Based on detailed core observation, thin section petrography combined with conventional wireline logs and core measurement analysis, 12 types of sedimentary- diagenetic facies were divided according to their lithofacies, diagenetic products and related contributions to reservoir quality. The identification and spatial distribution of these sedimentary-diagenetic facies can be determined based on the Bayes discriminant analysis and related cross-plot analysis. The variations of porosity and permeability with burial depth were precisely determined with ±2% for porosity and ±1 × 10–3 μm2 for permeability. The spatial distribution of reservoir porosity and permeability can be predicted quantitatively: high zones of porosity and permeability mainly distributed in the central sections of thick-bedded sandbody, ranging from 6% to greater than 12% for reservoir porosity and from 1 × 10–3 μm2 to greater than 6 × 10–3 μm2; relatively low zones of porosity (less than 6%) and permeability (less than 1 × 10–3 μm2) mainly occur in the bottom and the margin of the thick-bedded sandbody. [ABSTRACT FROM AUTHOR]

The Lower Permian Lucaogou Formation in the Jimsar sag, Junggar Basin is a typical tight-oil reservoir in China. For effective exploration and production, the formation of a high-quality reservoir must be thoroughly studied. In this work, the tight-oil reservoir was examined using a variety of methods, including core and thin-section observations, XRD, SEM, CL and fluid inclusion and isotope testing. The tight-oil reservoirs were primarily deposited in saline lake environments, which are dominated by variable admixture of dolomite, quartz, feldspar, tuff, calcite and pyrite. Nine main lithofacies were identified: (1) siliceous mudstone, (2) dolomitic siliceous mudstone, (3) dolomitic mudstone, (4) intraclast packstone/grainstone, (5) ooid grainstone, (6) bioclast grainstone, (7) dolomitic siltstone, (8) mixed siliclastic and intraclast grainstone and (9) brecciated dolomitic mudstone. The pore types are classified into four categories: primary intergranular, moldic, intercrystalline and fracture pores. The properties of tight-oil reservoirs are quite poor, with low porosity (ave. 7.85%) and permeability (ave. 0.110 mD) and a small pore-throat radius (ave. 0.086 μm). The tight-oil reservoirs are dominated by the aggradation of a repetitive meter-scale sedimentary facies succession that records distinct lacustrine expansions and contractions. These tight carbonates have also undergone significant diagenetic alterations, such as dolomitization, dissolution, neomorphism and fracture created intercrystalline and moldic pores, vug and fractures; chemical and mechanical compaction and carbonate cementation have decreased the reservoir quality. Variations in reservoir quality in the Jimusar sag are due to a combination of lithofacies type, high-frequency cyclic depositional architecture, dissolution intensity, dolomitization and tectonic related deformation. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Jimusar fine-grained rocks. [ABSTRACT FROM AUTHOR]

This paper investigates the reservoir potential of deeply-buried Eocene sublacustrine fan sandstones in the Bohai Bay Basin, China by evaluating the link between depositional lithofacies that controlled primary sediment compositions, and diagenetic processes that involved dissolution, precipitation and transformation of minerals. This petrographic, mineralogical, and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones. Eogenetic alterations of the sandstones include: 1) mechanical compaction; and 2) partial to extensive non-ferroan carbonate and gypsum cementation. Typical mesogenetic alterations include: (1) dissolution of feldspar, non-ferroan carbonate cements, gypsum and anhydrite; (2) precipitation of quartz, kaolinite and ferroan carbonate cements; (3) transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite. This study demonstrates that: 1) depositional lithofacies critically influenced diagenesis, which resulted in good reservoir quality of the better-sorted, middle-fan, but poor reservoir quality in the inner- and outer-fan lithofacies; 2) formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality; and 3) oil emplacement during early mesodiagenesis (temperatures > 70 °C) protected reservoirs from cementation and compaction. [ABSTRACT FROM AUTHOR]

• An optimal amount of low-permeable layers can increase the swept area of CO 2 plume. • An optimal amount of low-permeable layers can reduce the ratio of CO 2 in free phase. • Proximal-medial fluvial lithofacies associations are more suitable for CO 2 storage. Fluvial system deposits often form suitable reservoirs for CO 2 geological storage (CGS). These potential storage sites usually present heterogeneous fluvial architectures at multiple scales. This heterogeneity can exert varied effects on different aspects of CGS, resulting in significant storage efficiency variability. Here, we investigate the effects of variable fluvial lithofacies associations on CO 2 storage, using the Puig-reig anticline in the SE Pyrenees (Spain) as a reservoir analog. To test this, we employ a multidisciplinary approach that integrates field sedimentology, reservoir modeling, and numerical simulation of CO 2 injection to produce models akin to different fluvial lithofacies associations. The storage volume and injectivity of CO 2 are found to decrease in reservoirs with decreasing fractions and sizes of high-permeable facies from the proximal to the medial-distal lithofacies associations. The flow barriers created by low-permeable facies can hinder the vertical migration of the CO 2 plume and prevent it from reaching the reservoir top, hence reducing the direct contact between the CO 2 plume and the overlying caprock. Furthermore, an optimal amount of low-permeable layers (around 30% in this study) can increase the swept area of CO 2 and reduce the proportions of free CO 2 phase. These aspects can collectively increase the amount of permanently trapped CO 2 and reduce the leakage risks of the injected CO 2. Based on the characteristics of the resulting models (i.e., storage volume, injectivity, distribution and phases of CO 2), a multi-criteria decision-making method has been used to quantitatively rank the different lithofacies associations according to their suitability for CO 2 storage. In this analysis, the proximal-medial fluvial lithofacies associations are assessed to be the most suitable ones because they feature low proportions of the injected CO 2 reaching the reservoir top and in free phase while maintaining the high storage volume and injectivity. This study reveals that heterogeneous reservoir architectures have mixed effects on CO 2 storage, and that reservoirs featuring moderately heterogeneous architectures (i.e., fractions of low-permeable facies ranging from 30% to 40%) are beneficial to keeping the balance among different aspects of CO 2 storage. This provides new insights for the screening and selection of potential geological sites for CO 2 storage. [ABSTRACT FROM AUTHOR]

The Ordovician carbonate fault-controlled reservoir in the Shunnan area is substantially different from the typical karst reservoir closely related to an unconformity. The development of fault-controlled reservoirs is controlled by the deep and large strike-slip fault in the Middle Caledonian period. The characteristics, diagenesis types, diagenetic fluid evolution characteristics, fluid-rock interactions, and reservoir response of fault-controlled reservoirs were investigated systematically by analyzing 3D seismic, logging, and drilling core data. Production data, reservoir petrology, and chemical filling geochemistry were also used. The formation and evolution of fault-controlled reservoirs are affected by the coupling of the structure, lithofacies, and fluid. The dolomitization in the quasi-syngenetic period in the early stage of Episode I in the Middle Caledonian provided the material basis for the development of fault-controlled reservoirs. The Mg-rich high-salinity thermal fluid in the late Caledonian and the Early Hercynian provided a constructive transformation for the development of fault-controlled reservoirs. In contrast, the transformation of reservoirs by the Si-rich magmatic thermal fluid in the late Hercynian was mainly destructive, and the action range of the deep thermal fluid was strongly controlled by strike-slip fault zones. The releasing bend and superimposed parts of different structural styles in the fault zone have high activity intensity. These are areas where dissolution and transformation of the fluid occur preferentially. Dolomite is often more conducive to the development of fault fracture zones and promotes the dissolution and transformation of the deep thermal fluid. Therefore, the strike-slip fault cutting through the basement causes the hydrothermal fluid to enter the fault, dissolving carbonate minerals and forming a hydrothermal-modified fault-controlled reservoir composed of fractures, dissolution pores, and dolomite intercrystalline pores. The results provide a new approach and direction for the study of different types of deep and ultra-deep carbonate reservoirs. • The fault zone controls the distribution of fault-controlled reservoirs. • The dolomitization and Mg-rich thermal fluid are factors affecting reservoirs. • Dolomite promotes fault development and thermal fluid dissolution. [ABSTRACT FROM AUTHOR]

Complex mineral composition, multiple depositional processes, and strong reservoir heterogeneity cause the complexity and uniqueness of a lacustrine shale oil reservoir. Understanding the storage space development and hydrocarbon occurrence model should facilitate the analysis of the shale oil reservoir and hydrocarbon accumulation in lacustrine shales. In this study, the storage space development model for different lithofacies and the modes of hydrocarbon occurrence in different storage space were analyzed in the Es4s–Es3x shale in the Jiyang Sub-basin, East China, based on thin-section and field emission scanning electron microscopy (FE-SEM) observations, X-ray diffraction (XRD) analysis, physical property testing, and geochemical analysis. Inorganic pores, particularly recrystallization intercrystalline pores in calcite, are the key matrix pores for lacustrine shale oil. However, the types and abundance of storage space are noticeably different in various lithofacies. The hydrocarbon occurrence is mainly manifested in three states: (A) free state in interlaminar fractures, structural fractures, and abnormal pressure fractures; (B) adsorbed state in organic pores, intercrystalline pores in pyrite, and floccule pores; and (C) free state in large pore spaces, including dissolution pores and recrystallization intercrystalline pores, which can form a continuous hydrocarbon accumulation. The hydrocarbon generation potential and thermal maturity are closely associated with the lithofacies and mineral composition. The matching mechanisms of pore formation as well as hydrocarbon generation, migration, and accumulation are favorable. Among the various lithofacies, the organic-rich calcareous shale has abundant storage space, high porosity, total organic carbon content, and hydrocarbon potential, making it a "sweet spot" for shale oil exploration. • Recrystallization intercrystalline pores in calcite are the main matrix storage space for lacustrine shale oil. • Storage space development models of the different lithofacies were established. • Hydrocarbon occurrence is mainly manifested in three states. • Organic-rich calcareous shale is the "sweet spot" for shale oil exploration. [ABSTRACT FROM AUTHOR]

Reservoir quality is the key parameter controlling shale oil enrichment. However, the complicated lithofacies, which control diagenesis, result in reservoir heterogeneity in the shale strata, creating challenges for reservoir prediction. Combining with the detail analysis of inorganic and organic geochemistry, characteristics of the main lithofacies and diagenetic alteration were investigated in Permian Lucaogou Formation, Jimusar sag, Junggar Basin. Laminated rocks with laminae combinations containing pyroclastic and terrigenous clastic laminae, terrigenous clastic and dolomite laminae, and pyroclastic and carbonate laminae were identified. Massive rocks mainly includes dolomicrite, dolomitic tuff, silt dolostone, dolomitic siltstone, and tuffaceous siltstone. Feldspar dissolution and carbonate cementation were the two dominant diagenetic types, further causing heterogeneity of the shale oil reservoirs. Feldspar dissolution generally occurred in laminated rocks containing pyroclastic laminae with low content of carbonate minerals (PL1), as well as in their adjacent interbedded siltstones. Carbonate cementation generally occurred in laminated rocks containing carbonate laminae, as well as in their adjacent interbedded siltstones. Whether in laminated and massive rocks, sparry carbonate crystals were also precipitated adjacent to organic matter and in microfractures. In the burial processes, enriched transition metal elements of V and Ti in pyroclastic laminae promote catalytic pyrolysis of organic matter. Abundant CO 2 produced during the catalytic pyrolysis promoted feldspar dissolution. Dissolution of sedimentary carbonate minerals by organic acids and CO 2 provided the material sources for secondary carbonate cements. It promoted the recrystallization and re-precipitation of sparry carbonate crystals adjacent to organic matter and in the terrigenous clastic laminae or interbedded siltstones. Ca2+, Mg2+ Sr2+, Fe2+, and Mn2+ produced by the transformation of smectite to illite in the pyroclastic laminae also provided Ca source for secondary carbonate cements. External diagenetic fluids providing materials along microfractures is another genesis of carbonate cements in the Permian Lucaogou Formation. • Petrographic features in the shale strata in Permian Lucaogou Formation were identified based on in situ analytical method. • Differential feldspar dissolution and carbonate precipitation generally occurred in different lithofacies. • Catalytic pyrolysis of organic matter in presence of transition metal elements in pyroclastic laminae promoted dissolution. • Development of carbonate cements in the shale strata were mainly provided by three material sources. [ABSTRACT FROM AUTHOR]

• Sedimentary and geochemical characteristics indicate a semi-closed and shallow saline lake. • The dolomite is a product of near-surface dolomitization of precursor calcite, while siliciclastic grains were transported via eolian. • A new depositional model for the Lucaogou Formation of Jimsar Sag. Fine-grained sedimentary rocks are well developed in a salinized lacustrine basin in the Junggar Basin, China. Understanding the sedimentary characteristics and process of these fine-grained sediments is essential to find the exploration target for tight oil. In this study, the sedimentary characteristics and forming mechanism of fine-grained sediments in the Permian Lucaogou Formation within the Jimsar Sag were investigated based on core and thin section observation, SEM combined with XRD and geochemical data, and eleven lithofacies were identified, including (1) brecciated dolomitic mudstone, (2) intraclastic conglomerate, (3) mixed siliciclastic and intraclastic grainstone, (4) dolomitic siltstone, (5) Carbonate packstone and grainstone, (6) dolomitic mudstone, (7) laminated algae-bearing tuffaceous mudstone, (8) massive tuffaceous mudstone, (9) turbidite dolomitic mudstone to siltstone, (10) pebbled sandstone, and (11) green mudstone and siltstone. The organic constituents in the fine-grained sedimentary rocks include sapropelic, exinite, vitrinite, inertinite and secondary macerals, which are dominated by unstructured algae and asphaltene. The fine-grained sedimentary rocks are characterized by high-frequency cycles composed of three successive facies associations: profundal, littoral–sublittoral and palustrine, which recorded distinct lacustrine expansion and contraction. Developed mud cracks, paleosols and vertical dissolution fractures in the fine-grained sedimentary rocks indicate that these facies recorded the depositional process in an ephemeral to perennial lake in an arid climate. Fluvio-deltaic and aeolian transport were main modes for transporting the siliciclastic component that ultimately accumulated in a subaqueous and subaerial environment. The dolomite in the littoral zone might develop with the evaporitic concentration of the lake water during the early diagenetic stage. The depositional processes in the profundal zone included suspension and turbidity currents. Multiphase volcanic activities affected the lithofacies and prompted the accumulation of organic matter. Understating the depositional process of the fine-grained sedimentary rocks in the Lucaogou Formation in the Jimsar Sag is helpful to understand the depositional environment in the salinized lacustrine basin in the Junggar Basin. [ABSTRACT FROM AUTHOR]

The upper fourth member of the Eocene Shahejie Formation (Es4s) is a typical example of a Bohai Bay Basin lacustrine shale. These sedimentary units are characterized by markedly different mineral compositions compared to marine siliciclastic shales. Thus, understanding the characteristics and sedimentary environment of lacustrine shales enables analysis of their origin, enrichment of organic matter (OM), distribution of organic rich shales, and exploration targets for shale oil and gas. Based on thin sections and field emission scanning electron microscope observations from well core samples, combined with X-ray diffraction and geochemistry, the mineralogy, lithofacies, organic and geochemical characteristics as well as the sedimentary environment of the Es4s shale are analyzed in this paper. The results show that, across the study area, the Es4s shale is mainly composed of calcite, clay minerals, and quartz, but also contains subordinate dolomite, feldspar, pyrite, and anhydrite. On the basis of mineral composition, total organic carbon (TOC) content and sedimentary structures, eight lithofacies are identified. The TOC content of samples ranges between 0.15% and 11.4%, with an average of 2.27%, while OM primarily consists of Type I kerogen. Based on paleoclimate and paleosalinity data as well as redox conditions and primary productivity, the sedimentary environment of the Es4s shale can be further divided into four intervals: Intervals I, II, III, and IV, and these four intervals have large differences, rather than persistently warm-damp climate and reducing condition. The evolution of these intervals is also analyzed in detail in this study, and the controls on lithofacies and OM accumulation are also discussed. The results of this study suggest that the development of mineral compositions and frequent vertical lithofacies associations are strongly controlled by the cyclic evolution of the sedimentary environment. Thus, the accumulation of OM occurs as the result of a number of inter-connected factors including primary productivity, terrestrial inputs, and variation in oxidizing-reducing conditions. Across a given interval, these factors can either work together, or one can play a leading role. [ABSTRACT FROM AUTHOR]